1. Field of the Invention
The present invention relates to a dispersion-compensating optical fiber and an optical transmission path. The present specification is based on patent applications filed in Japan (Japanese Patent Application No. 2000-54646, Japanese Patent Application No. 2000-159071, Japanese Patent Application No. 2000-216587, Japanese Patent Application No. 2000-241547 and Japanese Patent Application No. 2000-266169), and the contents described in said Japanese patent applications are incorporated as a part of the present specification.
2. Background Art
Optical communication systems that transmit at the 1.55 μm wavelength band (the so-called C-band: typically covering a range of about 1.53-1.57 μm) are used practically by combining single-mode optical fibers for transmission such as a “1.3 μm band dispersion single-mode optical fiber having nearly zero chromatic dispersion at a wavelength of 1.3 μm” or a “standard single-mode optical fiber”, and dispersion-compensating optical fibers.
For example, since the chromatic dispersion of a 1.3 μm single-mode optical fiber is roughly +17 ps/nm/km (positive chromatic dispersion) at a wavelength of 1.55 μm, when this is used to perform optical communications in the 1.55 μm wavelength band, considerable chromatic dispersion occurs. In contrast, since dispersion-compensating optical fibers have negative chromatic dispersion in which the absolute value in the 1.55 μm wavelength band is comparatively large, by combining these in the manner described above, the chromatic dispersion that occurs in ordinary 1.3 μm single-mode optical fibers extending, for example, for several tens of kilometers, can be compensated by a dispersion-compensating optical fiber having a comparatively short used length.
In addition, since the dispersion slope of a 1.3 μm single-mode optical fiber in the 1.55 μm wavelength band is roughly +0.06 ps/nm2/km (positive value), in order to compensate according to this dispersion slope with the chromatic dispersion, it is preferable to use a dispersion-compensating optical fiber having a negative dispersion slope. If dispersion slope can be compensated, this can also be used in applications involving transmission of a plurality of pulsed light having different wavelengths as in the manner of high-density wavelength division multiplexing transmission (DWDM transmission).
On the other hand, transmission characteristics deteriorate when non-linear effects occur in optical fibers. In the case of propagating high-power signal light in the manner of optical communication systems using wavelength division multiplexing transmission and an optical amplifier that have already been used practically in particular, due to the high power density, non-linear effects tend to occur easily, resulting in the need for technology that suppress non-linear effects.
Although methods for suppressing non-linear effects have been proposed, including a method in which the non-linear refractive index of the optical fiber is decreased by reducing the amount of Ge, F or other dopant doped to the core, and a method in which Brillouin scattering, which is one of the non-linear effects, is suppressed by changing the outer diameter of the optical fiber when drawing from the fiber base material, enlargement of the effective area (which may be abbreviated as Aeff) of the optical fiber is a particularly effective method.
However, in the above-mentioned dispersion-compensating optical fibers of the prior art, although such optical fibers have been developed which attempt to improve the so-called performance index (FOM; Figure of Merit), which indicates the amount of chromatic dispersion per unit loss, while also being able to compensate the dispersion slope, it has been difficult to simultaneously realize these characteristics along with enlargement of Aeff.
In addition, a system that performs optical communication in the so-called L-band (1.57-1.63 μm), which is of a longer wavelength than the C-band that has been used in the past, has recently been examined. Wavelength bands used for optical transmission at present or in the future are reaching or will reach a broad range of 1.45-1.63 μm, which includes the so-called S-band (1.45-1.53 μm).
Thus, although a dispersion-compensating optical fiber is required that is able to compensate the chromatic dispersion and dispersion slope of single-mode optical fibers for transmission in not only the C-band, but also other wavelength bands such as the S-band and L-band, conventional dispersion-compensating optical fibers have been unable to adequately satisfy this requirement. Consequently, these optical fibers have been inadequate particularly in applications to wavelength multiplexing, high-speed, long-distance transmission and so forth.
In addition, although large Aeff is also simultaneously required in these wavelength bands, conventional dispersion-compensating optical fibers were unable to accommodate this requirement as well.
Moreover, dispersion-compensated optical fibers are required to have single-mode propagation at the used wavelength band and bending loss that is small enough to allow practical use.
In addition, these dispersion-compensating optical fibers of the dispersion slope-compensating type have conventionally been incorporated in optical communication systems in the form of modules by being housed in, for example, a suitable case. Recently however, studies have been conducted that attempt to form the dispersion-compensating optical fiber itself into a cable and insert it into the transmission path, and several of these attempts have been reported.
This is because, if a dispersion-compensating optical fiber itself was able to be used as a transmission path, since it would be possible to eliminate the arranging space of the module, while also being able to substantially shorten the length of the optical fiber through which the optical signals are transmitted, the transmission characteristics of the overall system could be improved.
However, although reports have been made regarding dispersion-compensating optical fibers of the dispersion slope-compensating type that place the emphasis on compensation of chromatic dispersion and dispersion slope as well as reduction of loss, there have been no effective studies or reports made regarding enlargement of Aeff.
Suppression of non-linear effects is essential for achieving the faster speeds, longer distances and wavelength multiplexing described above, and in the case of inserting a dispersion-compensating optical fiber of the dispersion slope-compensating type in the form of a transmission path, there are cases in which it is difficult to achieve practical use unless Aeff is provided to an extent that is able to effectively suppress non-linear effects.
In addition, dispersion-compensating optical fibers of the prior art required that, for example, the refractive index of the core center be larger than that of an ordinary single-mode optical fiber, and had the problem of increasing the amount of dopant doped to the core center.
Normally, a core center is formed from quartz glass doped with a dopant such as germanium etc. that provides the action of increasing refractive index, while the cladding provided around an outer periphery of the core is formed from pure quartz glass or fluorine-doped quartz glass, etc.
In addition, the glass transition point of the quartz glass decreases proportional to the amount of dopant doped. Thus, if the amount of dopant doped increases, since the difference in viscosities of the core and cladding increases when the fiber base material is heated and melted to draw the optical fiber, the drawing rate and drawing temperature are restricted from the viewpoint of mechanical strength, thereby resulting in the problem of being unable to obtain a dispersion-compensating optical fiber with low loss.
In addition, if the refractive index of the core center is high, Aeff tends to decrease. This results in greater susceptibility to the occurrence of non-linear effects, thereby leading to the problem of deterioration of transmission characteristics.
However, in the dispersion-compensating optical fibers of the prior art, optical fibers realizing chromatic dispersion and dispersion slope compensating effects while enabling the refractive index of the core center to be comparatively small have been unable to be obtained, resulting in problems in terms of ease of production, low loss and non-linearity, etc.
In addition, if Aeff is enlarged in an example of a dispersion-compensating optical fiber of the prior art, the absolute value of chromatic dispersion tends to become smaller, resulting in the problem of the length for compensating single-mode optical fibers for transmission becoming excessively long.